The enzymatic depolymerization of polygalacturonic acid was studied in a continuous stirred UF membrane reactor. Experimental data were obtained in different runs with varying amounts of biocatalyst from 0.25 to 1.35 mg. The residence time (1.7 h), temperature (25 degreesC), and membrane molecular weight cutoff (30 kDa) were kept constant. The time course of the oligomeric product concentration in the output solution was correlated with a model equation, that was originally developed for batch reactions and modified for the continuous system. A new experimental method was used to detect the instantaneous mean molecular weight. Evidence was obtained that the enzymatic breakdown was almost complete within 2-3 h from the startup of the bioreaction. The proposed method proved to be sufficiently accurate and easy to employ for rapid on-line control of the industrial bioprocess.